70 PHYSIOLOGICAL CHEMISTRY. 



panied by the liberation of another substance containing nitrogen, which 

 is an element in the composition of albumen. We know that this actu- 

 ally occurs in the living body, and that such a nitrogenous substance 

 (urea) is discharged with the urine. Still this only gives a possi- 

 bility, but not a proof, that the other product of decomposition is a fat. 

 Furthermore, the appearance of fat in isolated drops and granules, in 

 the substance of glandular cells or degenerating muscular fibres, of 

 which we know so many instances, has been regarded as an indication 

 that the fatty material in these cases is formed on the spot from the 

 albuminous substance in which it is imbedded. But it is evident that 

 the substance of the cell or muscular fibre is permeable to serous fluids 

 containing saccharine ingredients, and that these may have been the 

 immediate source of the fatty deposit. Most of the other reasons 

 adduced in favor of the production of fat from albuminous matters 

 are open to similar objections. On the whole, it may be said, that 

 while we have no reason to discredit the possibility of a fatty trans- 

 formation of albuminous matters, the main source of oleaginous sub- 

 stances, in point of fact, over and above those contained in the food, is 

 to be found in the carbo-hydrates. 



Physiological Relations of Fat. The fatty substances of the body 

 are subservient to a variety of uses. Some of these uses are of a physi- 

 cal character, while others imply chemical changes which are evidently 

 of the first importance in nutrition, though as yet unknown in their 

 details. The first and most palpable function of the adipose tissue is a 

 mechanical one. It acts as a 'cushion to protect the neighboring parts 

 from injury, and to facilitate the movement of muscular organs. The 

 adipose layer in the subcutaneous tissue, in the soles of the feet and 

 the palms of the hands, between the voluntary muscles, about the eyeball 

 at the fundus of the orbit, and about the heart at the origin of the great 

 vessels, is mainly useful in this way. The plicae adiposae of the artic- 

 ular cavities have a similar mechanical function, and the sebaceous 

 secretion of the cutaneous glandules, by its oleaginous properties, pro- 

 tects the skin and hair from desiccation and preserves their pliability. 

 The fatty tissue is also important as a non-conductor of heat. It 

 envelops the subcutaneous parts like a blanket, and retains in the 

 system much of the animal heat which would otherwise be dissipated. 

 Its deposit in certain localities, as in the omentum, has no doubt a 

 special reference to the protection, in this respect, of the underlying 

 organs. In all these situations, however, the fat is an indifferent body 

 in its chemical relations. Throughout the system, wherever fat can be 

 recognized by the microscope in the form of distinct drops or globules, 

 it is evidently, for the time being, in a state of physiological inactivity, 

 being transported by the circulating fluids or retained on the spot as a 

 nutriment of reserve. In certain parts, where it is very abundant, as 

 in the marrow of the long bones, we can hardly attribute to it any 

 further value than this. But it has also other functions immediately 

 connected with the renovation of the tissues. This is shown by the 



